It is interesting to see the impact of CSS by displaying this in Digital Editions and in FBReader (which has no CSS support).

Yes, CSS support is the one major feature that is lacking in FBReader. Most run-of-the-mill fiction ebooks display ok without CSS, but I've had quite a few ebooks (fiction and non-fiction) that were unusable in FBReader because of no CSS.

I love the program and I realize that it is only one guy working on it, but I wish that some other programmer would help out and work on adding CSS. It would make FBReader a killer app.

I think CSS is especially important, considering that FBReader and DE are currently the only two programs capable of reading epub. Given the drawbacks of DE that I pointed out above, FBReader would be a better choice for a lot of users if it had CSS.